It is known that to achieve maximum transmission efficiency in radiofrequency telecommunications systems, and chiefly, in those using artificial satellites, each antenna is to be used for the simultaneous transmission or reception of two different signals, while keeping as low as possible ohmic losses and mutual interferences. Moreover, if the antenna is installed on board a satellite its weight and encumbrance must be reduced as much as possible.
A solution to this problem is that of using a double-reflector antenna having a subreflector capable of generating a reflection at the virtual focus for the main reflector for one frequency or polarization and at the same time of allowing the operation of a feed placed in the primary focus for a second frequency or orthogonal polarization. Of course, a new feed can be placed at the virtual focus.
This can be achieved if the subreflector is selective to the frequency or to the polarization of the received or transmitted signal.
In this way it is transparent at a certain frequency or polarization, allowing the operation of the feed placed at the primary focus, and is reflecting at another frequency or polarization, allowing the operation of the feed placed at the virtual focus.
Moreover, in the case the antenna is used on board a satellite, the structure must fulfil severe requirements of mechanical stiffness, thermal deformation and weight.
Its weight must be as light as possible and its stiffness must ensure mechanical resonance frequencies higher than a minimum value, depending on the nature of the vector and on the type of support used. That is to avoid vibrations detrimental to the antenna when placing the satellite in orbit. Finally, thermal distortions, depending on sun irradiation in the orbit, have to be kept within predetermined levels in order to ensure good electrical antenna performances in the whole range of thermal variations.
More particularly, in case of frequency selective subreflectors, in addition to normal electrical specifications of on-board antennas, a ratio between reflection and transmission frequency as low as possible is required.
That is due to the fact that the main reflector is optimized at a well-determined frequency, hence, the closer the operation frequencies to the optimal frequency, the better the electrical performances in the two bands used. Now, practical considerations, depending on the bandwidth of the transmitted signals, seem to indicate in 1.5 the lower limit obtainable for the ratio above.
So far, antenna systems have already been launched with frequency or polarization selective subreflectors such as those installed on board the Voyager spacecraft.
In this case, frequency selectivity has been obtained with a surface consisting of a plurality of dielectric layers on one of which a plane distribution of cross-like metallic elements with bidimensional periodicity has been fabricated.
Such elements are usually referred to as "crossed dipoles." Their dimensioning depends on the reflection frequency only. The properties of transparence are, on the contrary, due to the fact that, at the transmission frequency considered, the dielectric structure is practically transparent and the grid of metallic elements is inactive.
All the antennas of this kind, already placed in orbit, exhibit a ratio between relection and transmission frequency higher than 2. It is known in the literature (see e.g. "Multilayer frequency sensitive surface" L. W. Henderson et al, International Symposium on antennas and propagation--1982 Albuquerque (USA), pages 459-62) that lower ratios require the use of two grids of electromagnetically coupled conducting elements.
In such a way, by exploiting the interference effects between the two grids, it is possible to obtain an effect of total transmission at a frequency even considerably near the reflection one. The reflection frequency remains anyway dependent on the size of the conducting elements, which may have different shapes: crossed dipoles, rings, etc. The transmission frequency depends on the contrary on the distance between the two grids, which is proportional to the ratio between reflection and transmission frequencies.
Polarization selectivity of the antennas now in orbit is obtained by the use of surfaces composed of a plurality of dielectric layers on one of which there is a plane periodic distribution of parallel metallic stripes. In this way the reflection of electrical fields polarized parallely to the stripes and the transmission of orthogonally-polarized ones are obtained.
In all these antennas the desired electromechanical properties of the subreflector have been obtained by the use of convenient multilayer structures of composite materials, shaped like a plate or honeycomb; they form a convenient mechanical support to the reflecting metallic grid.
An obvious soluton to the problem of making an antenna with a low value of the ratio between the reflection and transmission frequencies and convenient for use on board the satellites could consist in fabricating on a mechanical support, as described above, two dichroic grids separated by a convenient number of dielectric layers. However, in this way one of the grids is close to the mechanical support, whose layers made of composite materials have a rather high dielectric constant generally higher than 3. It is known that this closeness entails the lowering of the reflection frequency of the dichroic grid, which can be compensated for only by an initial grid dimensioning for higher frequencies. This requirement makes the grid embodiment more difficult when the reflection frequency exceeds about 15 GHz.
This problem could be solved by separating the mechanical support from the set of the two grids by a dielectric layer with low dielectric constant and convenient thickness. Moreover, the obtained structure presents a number of disadvantages:
too high ohmic losses in the transmission band and negligible in the reflection band; that is due to the fact that in the transmission band electromagnetic fields have to cross the whole structure and hence also the mechanical support, whose thickness is rather considerable, to meet thermomechanical requirements, while in the reflection band electromagnetic fields are nearly completely reflected from the most external grid, therefore they do not undergo significant attenuations;
the dielectric layer with a low dielectric constant actually decouples from a thermal standpoint the mechanical support of the set of the two grids, in this way a bad behaviour in presence of thermal variations is to be expected.